While it is a common practice to use a low-resolution liquid-crystal display (LCD) panel to display network information and text messages in a mobile device, it is preferred to use a high-resolution display to browse rich information content of text and images. A microdisplay-based system can provide full color pixels at 50–100 lines per mm. Such high-resolution is generally suitable for a virtual display. A virtual display typically consists of a microdisplay to provide an image and an optical arrangement for manipulating light emerging from the image in such a way that it is perceived as large as a direct view display panel. A virtual display can be monocular or binocular.
The size of the beam of light emerging from imaging optics toward the eye is called exit pupil. In a Near-Eye Display (NED), the exit pupil is typically of the order of 10 mm in diameter. Further enlarging the exit pupil makes using the virtual display significantly easier, because the device can be put at a distance from the eye. Thus, such a display no longer qualifies as an NED, for obvious reasons. Head-Up Displays are an example of the virtual display with a sufficiently large exit pupil.
WO 99/52002 discloses a method of enlarging the exit pupil of a virtual display. The disclosed method uses three successive holographic optical elements (HOEs) to enlarge the exit pupil. In particular, the HOEs are diffractive grating elements arranged on a planar, light transmissive substrate 6, as shown in FIG. 1. As shown, light from an image source 2 is incident upon the first HOE, or H1, which is disposed on one side of the substrate 6. Light from H1, coupled to the substrate 6, is directed toward the second HOE, or H2, where the distribution of light is expanded in one direction. H2 also redirects the expanded light distribution to the third HOE, or H3, where the light distribution is further expanded in another direction. The holographic elements can be on any side of the substrate 6. H3 also redirects the expanded light distribution outward from the substrate surface on which H3 is disposed. The optical system, as shown in FIG. 1, operates as a beam-expanding device, which maintains the general direction of the light beam. Such a device is also referred to as an exit pupil extender (EPE).
The EPE, such as that shown in FIG. 1, results in color non-uniformity, thereby degrading the quality of the reproduced virtual image. The color non-uniformity is mainly due to the fact that light beams of different colors travel different paths in the substrate 6, as shown in FIG. 2. For illustration purposes, only two colors, represented by λ1 and λ2, are used to show the source of color non-uniformity in the prior art EPE, with λ1<λ2.
In FIG. 2, only two HOEs are used, but the source of color non-uniformity is the same when three or more HOEs are used. The first HOE, or H1, typically has a diffractive structure consisting of parallel diffractive fringes for coupling incident light into the substrate 6 and directing the light distribution within the substrate 6 toward the second HOE, or H2. The substrate 6 acts as a light guide to trap the light beams between its two surfaces mainly by means of total internal reflection (TIR). As shown in FIG. 2, the diffractive elements H1 and H2 are both disposed on the lower surface of the substrate 6. In such an optical device, TIR is complete only at the upper surface, because part of the light is diffracted out from the lower surface of the substrate toward the viewer's eye.
It is known that the diffraction angle inside the substrate 6 is governed by:sin(θi)−n sin(θm)=mλ/dwhere                d is the grating period of the diffractive element (here H1)        λ is the wavelength        n is the refractive index of the substrate        m is the diffraction order        θi is the angle of incident, and        θm is the angle of diffraction in mth order.As can be seen from Equation 1, the diffraction angle θm increases with wavelength λ. Thus, the diffraction angle θm1 is smaller than the diffraction angle θm2. As a result, the interval L between two successive TIRs also varies with wavelength. The interval L1 for λ1 is smaller than the interval L2 for λ2. Thus, the distribution of outgoing light in the η direction is not uniform for all wavelengths, although the grating structure can be designed so that the output is homogeneous for one wavelength. As can be seen in FIG. 2, the shorter wavelength λ1 experiences more “hits” than and λ2 on the diffractive elements H2. Consequently, more light of the shorter wavelength λ1 “leaks” out of the diffractive element H2 in the area near H1. In a display where three primary colors (red, green, blue) are used, an EPE of FIG. 2 will cause an uneven color distribution of the light exiting the diffractive grating structure of H2. Thus, the color may appear bluish on the near end and reddish on the far end, relative to H1. As the distance along the η direction increases, the uneven color distribution becomes more noticeable.        
It should be noted that light can “leak” out of the substrate 6 from the lower surface where H2 is located or from the upper surface. The distribution of outgoing light from the upper surface is similar to that from the lower surface.
It is advantageous and desirable to provide a method and system for improving the color uniformity in light distribution in an exit pupil extender.